Kinetics of G protein-mediated modulation of the potassium M-current in bullfrog sympathetic neurons

Regulation of KCNQ2/KCNQ3 current by G protein cycling: the kinetics of receptor-mediated signaling by Gq

Receptor-mediated modulation of KCNQ channels regulates neuronal excitability. This study concerns the kinetics and mechanism of M₁ muscarinic receptor–mediated regulation of the cloned neuronal M channel, KCNQ2/KCNQ3 (Kv7.2/Kv7.3). Receptors, channels, various mutated G-protein subunits, and an optical probe for phosphatidylinositol 4,5-bisphosphate (PIP₂) were coexpressed by transfection in tsA-201 cells, and the cells were studied by whole-cell patch clamp and by confocal microscopy. Constitutively active forms of Gα_q and Gα₁₁, but not Gα₁₃, caused a loss of the plasma membrane PIP₂ and a total tonic inhibition of the KCNQ current. There were no further changes upon addition of the muscarinic agonist oxotremorine-M (oxo-M). Expression of the regulator of G-protein signaling, RGS2, blocked PIP₂ hydrolysis and current suppression by muscarinic stimulation, confirming that the G_q family of G-proteins is necessary. Dialysis with the competitive inhibitor GDPβS (1 mM) lengthened the time constant of inhibition sixfold, decreased the suppression of current, and decreased agonist sensitivity. Removal of intracellular Mg²⁺ slowed both the development and the recovery from muscarinic suppression. When combined with GDPβS, low intracellular Mg²⁺ nearly eliminated muscarinic inhibition. With nonhydrolyzable GTP analogs, current suppression developed spontaneously and muscarinic inhibition was enhanced. Such spontaneous suppression was antagonized by GDPβS or GTP or by expression of RGS2. These observations were successfully described by a kinetic model representing biochemical steps of the signaling cascade using published rate constants where available. The model supports the following sequence of events for this G_q-coupled signaling: A classical G-protein cycle, including competition for nucleotide-free G-protein by all nucleotide forms and an activation step requiring Mg²⁺, followed by G-protein–stimulated phospholipase C and hydrolysis of PIP₂, and finally PIP₂ dissociation from binding sites for inositol lipid on the channels so that KCNQ current was suppressed. Further experiments will be needed to refine some untested assumptions.

Modulation and genetic identification of the M channel

Potassium channels constitute a superfamily of the most diversified ion channels, acting in delicate and accurate ways to control or modify many physiological and pathological functions including membrane excitability, transmitter release, cell proliferation and cell degeneration. The M-type channel is a unique ligand-regulated and voltage-gated K(+) channel showing distinct physiological and pharmacological characteristics. This review will cover some important progress in the study of M channel modulation, particularly focusing on membrane transduction mechanisms. The K(+) channel genes corresponding to the M channel have been identified and will be reviewed in detail. It has been a long journey since the discovery of M current in 1980 to our present understanding of the mysterious mechanisms for M channel modulation; a journey which exemplifies tremendous achievements in ion channel research and exciting discoveries of elaborate modulatory systems linked to these channels. While substantial evidence has accumulated, challenging questions remain to be answered.

Modulation of M-channel conductance by adenosine 5' triphosphate in bullfrog sympathetic B-neurones

1. Adenosine 5' triphosphate (ATP) (0.5-500 microM) or muscarine (0.1-1.0 microM) suppressed M-current/conductance (IM/gM) in B-cells of bullfrog sympathetic ganglion. Both agonists suppressed steady-state M-conductance (gM) at -30 mV and there was either no change or a slight increase in the time constants for gM activation (tau(a) at -30 mV) and deactivation (tau(d) at -50 mV). 2. It has previously been shown that experimental elevation of intracellular Ca2+ concentration ([Ca2+]i) suppresses gM and this is associated with decreases in both tau(a) and tau(d). As these changes in kinetics differ from those we observe with agonist application, our results cast doubt on the hypothesis that elevation of [Ca2+]i is involved in the transduction mechanism for ATP- or muscarine-induced gM suppression.

A novel slow hyperpolarization-activated potassium current (IK(SHA)) from a mouse hippocampal cell line

1. A slow hyperpolarization-activated inwardly rectifying K+ current (IK(SHA)) with novel characteristics was identified from the mouse embryonic hippocampus x neuroblastoma cell line HN9.10e. 2. The non-inactivating current activated negative to a membrane potential of -80 mV with slow and complex activation kinetics (tau act approximately 1-7 s) and a characteristic delay of 1-10 s (-80 to -140 mV) that was linearly dependent on the membrane potential. 3. Tail currents and instantaneous open channel currents determined through fast voltage ramps reversed at the K+ equilibrium potential (EK) indicating that primarily K+, but not Na+, permeated the channels. 4. IK(SHA) was unaffected by altering the intracellular Ca2+ concentration between approximately 0 and 10 microM, but was susceptible to block by 5 mM extracellular Ca2+, Ba2+ (Ki = 0.42 mM), and Cs+ (Ki = 2.77 mM) 5. In cells stably transformed with M2 muscarinic receptors, IK(SHA) was rapidly, but reversibly, suppressed by application of micromolar concentrations of muscarine. 6. At the single channel level K(SHA) channel openings were observed with the characteristic delay upon membrane hyperpolarization. Analysis of unitary currents revealed an inwardly rectifying I-V profile and a channel slope conductance of 7 pS. Channel activity persisted in the inside-out configuration for many minutes. 7. It is concluded that IK(SHA) in HN9.10e cells represents a novel K+ current, which is activated upon membrane hyperpolarization. It is functionally different from both classic inwardly rectifying IKir currents and other cationic hyperpolarization-activated IH currents that have been previously described in neuronal or glial cells.

Na(+)-Ca2+ exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate

Na(+)-Ca2+ exchanger-associated membrane currents were studied in cultured murine neocortical neurons, using whole-cell recording combined with intracellular perfusion. A net inward current specifically associated with forward (Na+(o)-Ca2+(i)) exchange was evoked at -40 mV by switching external 140 mM Li+ to 140 mM Na+. The voltage dependence of this current was consistent with that predicted for 3Na+:1Ca2+ exchange. As expected, the current depended on internal Ca2+, and could be blocked by intracellular application of the exchanger inhibitory peptide, XIP. Raising internal Na+ from 3 to 20 mM or switching the external solution from 140 mM Li+ to 30 mM Na+ activated outward currents, consistent with reverse (Na+(i)-Ca2+(o)) exchange. An external Ca2(+)-sensitive current was also identified as associated with reverse Na(+)-Ca2+ exchange based on its internal Na+ dependence and sensitivity to XIP. Combined application of external Na+ and Ca2+ in the absence of internal Na+ triggered a 3.3-fold larger inward current than the current activated in the presence of 3 mM internal Na+, raising the intriguing possibility that Na(+)-Ca2+ exchangers might concurrently operate in both the forward and the reverse direction, perhaps in different subcellular locations. With this idea in mind, we examined the effect of excitotoxic glutamate receptor activation on exchanger operation. After 3-5 min of exposure to 100-200 microM glutamate, the forward exchanger current was significantly increased even when external Na+ was reduced to 100 mM, and the external Ca2(+)-activated reverse exchanger current was eliminated.

Exotoxin-insensitive G proteins mediate synaptically evoked muscarinic sodium current in rabbit sympathetic neurones

1. The involvement of G proteins in the transduction pathway that links muscarinic receptors to the low-threshold voltage-dependent sodium current (INa,M) was studied in neurones from intact sympathetic prevertebral ganglia using the whole-cell configuration of the patch-clamp technique. Experiments were performed in the presence of the nicotinic receptor antagonists hexamethonium (50 microM) and d-tubocurarine (50 microM). 2. INa,M was activated by either bath-applying muscarinic agonists or stimulating the preganglionic splanchnic nerves. Synaptically and agonist-mediated INa,M did not display significant run-down or changes in their properties in cells tested, irrespective of whether the pipette solutions contained GTP. 3. Dialysis of sympathetic neurones with GDP beta S (500-750 microM) decreased the amplitude of INa,M by approximately 65% compared with control neurones within 30 min. 4. In the absence of muscarinic receptor stimulation, intracellular dialysis with GTP gamma S (500 microM) for 10 min slowly and slightly (20-25%) activated INa,M. GTP gamma S dialysis markedly slowed down the decay of INa,M after its transient activation with carbachol pulses (10-20 s) or nerve stimulation (3-5 s). The INa,M activation became fully irreversible 2.9 min after the start of GTP gamma S dialysis. Dialysing cells with the G protein activator AIF4-led to a rapid but transient activation of INa,M. 5. Synaptically and agonist-evoked INa,M were not affected in ganglia treated with 0.5-1 microgram ml-1 pertussis toxin (PTX) for 7-24 h at 37 degrees C. Control experiments showed that this treatment severely reduced the PTX-sensitive inhibition of N-type calcium currents induced by carbachol (CCh) and noradrenaline. Application of NEM (N-ethylmaleimide) for 2 min depressed the INa,M evoked in response to bath-applied CCh by only 27%. 6. Incubating ganglia with 5-10 micrograms ml-1 of cholera toxin for 7 h had no effect on the carbachol-induced INa,M but greatly potentiated (approximately 250%) the synaptically evoked INa,M, presumably via a presynaptic mechanism. 7. These results show that the coupling between muscarinic receptors and NaM channels is mediated by pertussis toxin- and cholera toxin-insensitive G proteins, possibly of the Gq/11 or G12 class.

Control of M-current

M-current is a non-inactivating potassium current found in many neuronal cell types. In each cell type, it is dominant in controlling membrane excitability by being the only sustained current in the range of action potential initiation. It can be modulated by a large array of receptor types, and the modulation can occur either by suppression or enhancement. Modulation of M-current has dramatic effects on neuronal excitability. This review discusses the numerous second messenger pathways that converge on regulation of this current: in particular, two forms of regulation of the M-current, receptor-mediated modulation and the control of macroscopic current amplitude by intracellular calcium. Both types of regulation are discussed with reference to the modulation of single-channel gating properties.

Muscarinic mechanisms in nerve cells

The receptor subtype and transduction mechanisms involved in the regulation of various neuronal ionic currents are reviewed, with some recent observations on sympathetic neurons, hippocampal cell membranes and basal forebrain cells.

Methods for studying neurotransmitter transduction mechanisms

This review describes the methodologies used to study the transduction mechanisms that are activated in excitable cells by G-protein-coupled agonists. In view of the complexity of second-messenger systems, it is no longer relevant to ask, "What is the transduction mechanism involved in the action of a given neuromodulator?" because, in many cases, a variety of transduction mechanisms and physiological responses are invoked following receptor activation. This means that a single aspect of the physiological response must be selected for study in order to address the question of transduction mechanism. This review is therefore concerned with a description the use of patch- and voltage-clamp procedures to study transduction mechanism because they are designed to isolate one aspect of the physiological response: the change in activity of a single type of membrane ion channel.

Delayed onset and slow time course of the non-M-type muscarinic current in bullfrog sympathetic neurons

The onset and time course of the muscarinic currents induced by brief applications of acetylcholine (ACh) were examined in voltage-clamped neurons of bullfrog sympathetic ganglia bathed in a solution containing d-tubocurarine. At a potential of -40 mV, the ACh-induced current (IACh) appeared within 1.2 s and rapidly increased to its peak with a half-activation time of 2.2 s. This initial current was termed the fast IACh and was blocked by 4 mM Ba2+. At a potential more negative than -60 mV, the fast IACh disappeared and the remaining IACh activated with a delay of 3.9 s and slowly increased to its peak with a half-activation time of 8.2 s. This delayed current was termed the slow IACh and is thought to be associated with inhibition of a K+ current, or IM, as well as activation of an inward current through non-M-type muscarinic cation channels. The slow IACh was not inhibited by Ba2+, but its amplitude was reduced with depolarization (the extrapolated reversal potential was +3 mV). In Na(+)-free solution, the amplitude of the slow IACh reduced, but its polarity did not reverse in the voltage region examined (-30 to -100 mV). The slow excitatory postsynaptic current was also recorded, and was shown to have a similar delay in onset and slow time course. The results demonstrate that ACh activates the non-M-type muscarinic current three times more slowly than it inhibits IM.

Effects of muscarine on K(+)-channel currents in the C-cells of bullfrog sympathetic ganglion

The effects of muscarine on small, putative C-cells and large, putative B-cells dissociated from bullfrog paravertebral sympathetic ganglia were studied by whole cell and single channel recording techniques. The dominant action of muscarine was to activate an inwardly-rectifying K+ current (IK(G)) in C-cells and to suppress M-current (IM) in B-cells. However, both IM and IK(G) were affected by muscarine in 5 out of 78 putative C-cells and in 8 others only IM was affected. By contrast, IK(G) was only activated in 1 out of 105 B-cells. This predicts that the muscarinic slow IPSP, which can be evoked by preganglionic stimulation, occurs exclusively in C-cells. 6% of these cells could, however, generate a muscarinic slow EPSP in addition to a slow IPSP and 10% could generate a slow EPSP without a slow IPSP. The rectification associated with IK(G) was neither a direct consequence of the direction of movement of K+ ions nor a simple consequence of channel block by intracellular Mg2+ or Na+ ions. The fit of the activation curve by a Boltzmann equation suggests that the conductance underlying IK(G) is controlled by a voltage-dependent gating charge (valency approximately -2). Muscarine activated no new channels in outside-out or cell-attached patches but increased the opening probability of two types of K+ channels (unitary conductances approximately 20 pS and approximately 55 pS). The possible role of these channels in the generation of IK(G) is discussed.

Phorbol ester-induced M-current suppression in bull-frog sympathetic ganglion cells: insensitivity to kinase inhibitors

1. The effects of 1-oleoyl-2-acetyl-sn-glycerol (OAG), phorbol 12-myristate 13-acetate (PMA), 4-alpha-phorbol and muscarine on B-neurones from bull-frog sympathetic ganglion were studied by means of whole-cell patch-clamp recording. With the exception of 4-alpha-phorbol, all of these agonists reduced the steady-state outward current recorded at -30 mV as a result of suppression of a voltage-dependent, non-inactivating K(+)-current, the M-current, (IM). 2. Of the cells tested, 34% displayed bona fide responses to OAG (20 microM). The chance of recording a response was not decreased when the protein kinase inhibitor, 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7; 50 or 75 microM) was included simultaneously in the extracellular solution and in the pipette solution. 3. The presence of 50 microM H-7 on both sides of the membrane or 500 nM staurosporine in the pipette solution did not prevent responses to brief (1-2 min) or prolonged (> 20 min) applications of PMA. 4. Brief (1-2 min) extracellular application of H-7 (300 microM) suppressed IM by about 29%. 5. The most likely explanation of these data is that PMA and OAG modulate IM via a mechanism that is independent of protein kinase C (PKC). The availability of such a mechanism poses new questions as to the mechanism of muscarine-induced IM suppression.

Synthesis of models for excitable membranes, synaptic transmission and neuromodulation using a common kinetic formalism

Markov kinetic models were used to synthesize a complete description of synaptic transmission, including opening of voltage-dependent channels in the presynaptic terminal, release of neurotransmitter, gating of postsynaptic receptors, and activation of second-messenger systems. These kinetic schemes provide a more general framework for modeling ion channels than the Hodgkin-Huxley formalism, supporting a continuous spectrum of descriptions ranging from the very simple and computationally efficient to the highly complex and biophysically precise. Examples are given of simple kinetic schemes based on fits to experimental data that capture the essential properties of voltage-gated, synaptic and neuromodulatory currents. The Markov formalism allows the dynamics of ionic currents to be considered naturally in the larger context of biochemical signal transduction. This framework can facilitate the integration of a wide range of experimental data and promote consistent theoretical analysis of neural mechanisms from molecular interactions to network computations.

Cyclic ADP-ribose modulates Ca2+ release channels for activation by physiological Ca2+ entry in bullfrog sympathetic neurons

Although Ca(2+)-induced Ca2+ release (CICR) via ryanodine receptors has been found to occur in intact neurons, little is known about the physiological processes that regulate it. We studied the effects of cyclic ADP-ribose (cADPR) on CICR in cultured bullfrog sympathetic neurons by fura-2 fluorescence recording and patch-clamp techniques. cADPR applied through a patch pipette augmented action potential- or depolarizing pulse-induced rises in intracellular Ca2+ without a change in Ca2+ entry initiating the responses, but not in the presence of ryanodine. Likewise, cADPR enhanced a single or oscillatory rise(s) in intracellular Ca2+ induced by caffeine. These results strongly suggest that cADPR can be an endogenous modulator of ryanodine receptors in neurons.

On the mechanism of M-current inhibition by muscarinic m1 receptors in DNA-transfected rodent neuroblastoma x glioma cells

1. Acetylcholine (ACh) produces two membrane current changes when applied to NG108-15 mouse neuroblastoma x rat glioma hybrid cells transformed (by DNA transfection) to express m1 muscarinic receptors: it activates a Ca(2+)-dependent K+ conductance, producing an outward current, and it inhibits a voltage-dependent K+ conductance (the M conductance), thus diminishing the M-type voltage-dependent K+ current (IK(M)) and producing an inward current. The present experiments were undertaken to find out how far inhibition of IK(M) might be secondary to stimulation of phospholipase C, by recording membrane currents and intracellular Ca2+ changes with indo-1 using whole-cell patch-clamp methods. 2. Bath application of 100 microM ACh reversibly inhibited IK(M) by 47.3 +/- 3.2% (n = 23). Following pressure-application of 1 mM ACh, the mean latency to inhibition was 420 ms at 35 degrees C and 1.79 s at 23 degrees C. Latencies to inhibition by Ba2+ ions were 148 ms at 35 degrees C and 92 ms at 23 degrees C. 3. The involvement of a G-protein was tested by adding 0.5 mM GTP-gamma-S or 10 mM potassium fluoride to the pipette solution. These slowly reduced IK(M), with half-times of about 30 and 20 min respectively, and rendered the effect of superimposed ACh irreversible. Effects of ACh were not significantly changed after pretreatment for 24 h with 500 ng ml-1 pertussis toxin or on adding up to 10 mM GDP-beta-S to the pipette solution. 4. The role of phospholipase C and its products was tested using neomycin (to inhibit phospholipase C), inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4), heparin, and phorbol dibutyrate (PDBu) and staurosporin (to activate and inhibit protein kinase C respectively). Both neomycin (1 mM external) and InsP3 (100 microM intrapipette) inhibited the ACh-induced outward current and/or intracellular Ca2+ transient but did not block ACh-induced inhibition of IK(M). Intrapipette heparin (1 mM) blocked activation of IK(Ca) and reduced Ach-induced inhibitions of IK(M), but also reduced inhibition of ICa via endogeneous m4 receptors. PDBu (with or without intrapipette ATP) and staurosporin had no significant effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective reduction of one mode of M-channel gating by muscarine in sympathetic neurons

M-current is widespread in the nervous system. It stabilizes cell excitability, and its suppression by muscarinic receptor activation underlies slow synaptic transmission in sympathetic neurons. Suppression of M-current was one of the first examples of neuromodulation of a potassium current, but the mechanism is not understood. Single-channel recording was used to study this issue. An M-channel with two conductance states, which exhibited appropriate voltage-dependent kinetics with two modes of gating, has been resolved. Mode 1 comprises short open time, low open probability events, and mode 2 openings represent long open time, high open probability behavior. Muscarine decreased M-channel activity by selectively reducing mode 2 M-channel gating through a diffusible second messenger. It is suggested that control of modal gating may be a widespread mechanism for neuromodulation.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Closure of potassium M-channels by muscarinic acetylcholine-receptor stimulants requires a diffusible messenger

The M-current (IK(M)) is a slow voltage-gated K+ current which can be inhibited by muscarinic acetylcholine-receptor (mAChR) agonists. In the present experiments we have tested whether this inhibition results from a local (membrane-delimited) interaction between the receptor and adjacent channels, or whether channel closure is mediated by a diffusible messenger. To do this, single KM(+)-channel currents were recorded from membrane patches in dissociated rat superior cervical sympathetic neurons by using cell-attached patch electrodes. Channel activity was inhibited when muscarine was applied to the cell membrane outside the patch but persisted when channels were exposed to muscarine added to the pipette solution. We conclude that a diffusible molecule (or molecules) is (are) required to induce intrapatch channel closure following activation of extra-patch receptors.

The effect of arachidonic acid on the M current of NG108-15 neuroblastoma x glioma hybrid cells

The M current, IM, a voltage-dependent non-inactivating K+ current, was recorded in NG108-15 neuroblastoma x glioma hybrid cells, using the whole-cell mode of the patch-clamp technique. We studied the effect of arachidonic acid, other fatty acids and inhibitors of the arachidonic acid metabolism. In relatively high concentrations (25-50 microM) arachidonic acid first increased and later decreased the current, Ih, which holds the membrane potential at -30 mV and mainly flows through open M channels. It shifted the midpoint potential, Vo, of the relation between M conductance, gM, and membrane potential, V, to more negative values and decreased the maximum conductance gM and the time constant tau M. In smaller concentrations (5-10 microM) arachidonic acid merely decreased Ih and gM with little effect on Vo and tau M. Eicosatetraynoic acid and docosahexaenoic acid acted similarly to arachidonic acid whereas stearic acid had no effect. Of the three enzyme inhibitors studied, nordihydroguaiaretic acid acted similarly to arachidonic acid. i.e. caused a biphasic change in Ih. Indomethacin and quinacrine caused, respectively, a pure increase and a pure decrease of Ih and gM. Possible explanations are build-up of internally produced arachidonic acid, depletion of eicosanoid products or an inhibitory effect unrelated to arachidonic acid metabolism.

Neuromodulation

The ability of the nervous system to respond to the environment and to learn depends upon the tuning of neuronal electrical activity, loosely called neuromodulation. The substrates for electrical activity and, therefore, neuromodulation are ion channels which may be either synaptic or extrasynaptic. Neuromodulation is dynamic and most frequently involves neurotransmitters and hormones acting via G-protein-coupled pathways.